Is horse poop good fertilizer for petunias and dianthus

In a small pot (40-45cm), I want to know if some horse poop is needed to grow 5 petunias and 1 dianthus. If so, I can get some to use in the pot to grow some better plants. The pot is covered by some plants - I don't know the name - to give it some shade but still gets some good sunlight. It was planted on the 11th April 2015 from Bunings in Townsville for fun. I use to water it with normal tap water once a day but now I water the plant once every two or three days with normal tap water. Please answer my question.